1. Field of the Invention
The present invention relates to a night vision goggle compatible color liquid crystal display. In particular, the present invention relates to a night vision goggle compatible color liquid crystal display having liquid crystal cells arranged in a matrix, in which three adjacent cells are respectively assigned red, green and blue filters and can be driven by a control device with red, green and blue pixel information. The brightness of a backlight source can be switched from a high value during a daytime operation to a low value during a nighttime operation.
2. Description of Related Art
A color liquid crystal display for night vision goggles are known, for example, from U.S. Pat. No. 5,262,880.
Night vision goggles (NVGs) are used in aircraft, particularly military aircraft, to enable the pilot to see out of the aircraft at night. To monitor instruments and displays in the cockpit, the pilot looks past the night vision goggles either below or on the side. Thus, a night vision goggle compatible display requires that it can be seen with the naked eye but is invisible through the night vision goggles.
Night vision goggles convert radiation in the near infrared region of the spectrum into visible light. A so-called minus blue filter is used to block wavelengths shorter than 625 nm in Class A night vision goggles (A NVGs) and wavelengths shorter than 665 nm in Class B night vision goggles (B NVGs). Thus, for the liquid crystal display not to interfere with the NVGs, during the nighttime operation a display must not emit radiation with a wavelength longer than 625 nm for A NVGs or 665 nm for B NVGs.
To ensure that the display emits proper radiation, a respective infrared absorbing filter is provided in the display. Because the 625 nm wavelength is between the visible colors red and orange, red indicator lamps and color displays are not compatible with Class A night vision goggles. In contrast, Class B night vision goggles allow the use of red indicator lamps and color displays in the cockpit because the 665 nm wavelength is approximately in the middle of the red wave length range. In the case of Class B night vision goggles, however, the infrared absorbing filter in the color display must have the steepest possible filtering slope near the 665 nm wavelength, such that the remaining red components of the shorter wavelengths are blocked as little as possible and distortions in the color rendition are largely avoided. Filters of this type with a steep filtering slope—typically dichroic filters—are completely effective only if the radiation passes essentially perpendicularly through the filters.
The display, on the other hand, should be readable within the widest possible viewing angle range. Thus, the color liquid crystal display disclosed in the aforementioned U.S. Pat. No. 5,262,880 uses a collimator to guide the light emitted by the backlight source perpendicularly through the infrared absorbing filter and a diffuser to subsequently diffuse the light uniformly before it reaches the matrix with the liquid crystal cells. Despite these measures, however, the fact remains that, during both nighttime and daytime operations of the color display, the longer-wave red components are blocked and the color rendition is thus distorted.
U.S. Pat. No. 5,661,578 proposes using two different backlight assemblies, one for the nighttime operation and the other for the daytime operation of the color liquid crystal display. In this two backlight assembly, only the backlight assembly for the nighttime operation has an infrared absorbing filter, so that color distortion is excluded at least during the daytime operation of the display.